The present invention relates broadly to a troposcatter radio system and in particular to a tropo system channel distortion weighting apparatus for predetection combiners.
The method or system of transmitting microwaves within the troposphere to effect radio communication between two points on the earth's surface has been utilized in many areas of intelligence communications. The troposphere is the lower layer of the earth's atmosphere which extends to about 60,000 feet at the equator and 30,000 feet at the poles. The use of tropospheric scattering provides the means for communicating over moderate distances of from 70 to 600 miles. However, such a span may be augmented by other spans in tandem to permit end to end or through circuits up to many thousand miles. Thus, the maximum range for wireless communications at frequencies from several hundred megacycles on up into the microwave no longer depends on the line of sight distance between the transmitter and receiver which was severely limited by both the earth's curvature and intervening physical obstacles, such as mountainous terrain. However, it is well known that maximal ratio predetection diversity combining is optimum for channels subject to flat (non-frequency selective) amplitude and phase fading. On most practical links of interest such as troposcatter, the classical flat fading channel model is not appropriate. Instead, significant channel distortion is experienced such that a significant probability exists that a channel will have both a high signal to thermal noise ratio and high signal distortion. The present invention solves the problem of capture by a strong but distorted signal and weighs each of the four input IF channels according to the ratio of undistorted signal to signal distortion plus thermal noise.